Microbuckle initiation from a hole and from the free edge of a fibre composite

Abstract

Microbuckle initiation from an open hole, from a filled hole, and from the free surface of a unidirectional fibre composite is examined by a finite element method. The role of fibre bending resistance is included in the formulation so that a hole size effect is predicted. It is found that the open hole knocks-down the compressive strength to the order of 20 τ Y, while the filled hole reduces the strength to about 40 τ Y, where τ Y is the in-plane shear modulus of the composite. Pre-existing fibre waviness adjacent to the hole gives a further reduction in the compressive strength. The role of in-plane shear stress and transverse stress in reducing the open hole compressive strength is examined: the compressive strength is reduced by the presence of shear and transverse tension, but the effect is less pronounced than that observed previously for the case of a circular patch of waviness. The study concludes with an investigation of the influence of the free surface on microbuckle initiation from a band of waviness and from a semi-circular patch of waviness: a negligible effect of the free surface on the compressive strength is predicted.